Transflective liquid crystal displays, in particular transflective active matrix liquid crystal displays (AM-LCD) and color super twisted nematic liquid crystal displays (CSTN-LCD), are commonly used in mobile handheld applications. Such display devices are preferred due to their comparatively low power consumption and their good front-of-screen performance. A transflective liquid crystal display is characterized by its ability to function in a reflective mode enabling reflection of ambient light, as well as in a transmissive mode enabling transmission of light from an auxiliary light source (for example from a backlight). Devices provided with such transflective displays therefore provide acceptable readability under bright as well as dark conditions.
The fundamental principles of transflective displays are well known, for example from the UK patent application 2 101 347. A transflective liquid crystal display thus comprises a display cell having a front substrate and a rear substrate between which a layer of liquid crystal (LC) material is sandwiched. In front of the display cell a front polarizer, or analyzer, and a compensation film is sandwiched.
Furthermore, an optical rear stack comprising a quarter-wave retarder and a linear polarizer element, a diffusing layer, and a light source, is sandwiched in said order behind the rear substrate. Finally, a transflector is arranged between the optical rear stack and the liquid crystal layer.
Transflective displays can operate according to a normally black cell concept or a normally white cell concept. A normally black cell concept provides black images in case no electric field is applied across the liquid crystal and a normally with cell concept provides a bright image in case no electric filed is applied.
When a normally black cell is operating in the reflective mode, and when the cell is inactivated (i.e. no electric field is applied across the LC layer), ambient light entering the front substrate will be absorbed due to the alignment of directors in the liquid crystal layer. A viewer then perceives the display cell as being dark. On the other hand, when the cell is activated (i.e. an electric field is applied between the electrodes), the directors in the liquid crystal are dislocated and instead essentially perpendicular to the front and rear substrates. Thereby a substantial part of the ambient light entering the front substrate is allowed to pass through the cell, reflect at the transflector, and return towards the front substrate. A viewer then perceives the display as being bright.
However, when the ambient light is insufficient for the display to emit a readable picture the light source is turned on activating the transmissive mode. When the transmissive mode is active, randomly polarized light emitted from the light source is linearly polarized by the polarizer and circularly polarized by the quarter-wave retarder (in the optical rear stack), and transmitted through the transflector. In case the liquid crystal cell is inactive, light impinging thereon from the light source is however absorbed in the front liquid crystal. The inactive cell is therefore perceived as dark. On the other hand, when the cell is activated and the liquid crystal molecules are perpendicular to the substrates, a substantial part of the light transmitted through the transflector will exit the cell towards the viewer, setting the display in a bright mode.
The operation of a normally white cell is much the same as for a normally black cell with suitable modifications. Several other arrangements are disclosed in the literature, using configurations similar to the one described above.
Regardless of the particular application at hand, it is very important for the light transmitted towards the liquid crystal layer to have accurate polarization. Light having incorrect polarization will not be affected by the aligned directors in the liquid crystal layer and will thus be transmitted through the liquid crystal regardless of its state (i.e. aligned or disturbed directors). In the following, accurately polarized light transmitted towards the LC layer is regarded as constructive light for the display while light having wrong polarization transmitted towards the LC layer is regarded as destructive light for the display.
In current commercial products, typical reflection/transmission ratios of the transflector vary from 90/10 to 60/40. This means that 60-90% of the light emitted from the auxiliary light source (i.e. the backlight) is not transmitted. This is one of the reasons why typical transmission values (ratios) of transflective liquid crystal displays are very low (2-3%) resulting in low brightness. Separate adjustments and optimizations of the reflective and transmissive mode performances are available, improving the performance but also resulting in manufacturing difficulties and higher production costs. And still, a trade-off still has to be made in the transflective display between transmissive and reflective performance.
Basically, 90 percent of the light incident from the light source is wasted at a 90/10 transflector. This means that the desired output power needs to be increased ten times only to compensate for the poor transmissivity of the transflector. The light source is typically among the most power consuming parts in a transflective display. Increasing the fraction of constructive light transmitted through the transflector, without affecting the reflective properties, is therefore highly desired.